Method and apparatus for electroplating a semiconductor wafer

ABSTRACT

A method, apparatus and anode for plating copper or other metals onto a barrier or seed layer of a wafer surface is described. A copper layer of uniform thickness is plated on the surface by, for instance, maintaining a constant current density between the anode and wafer surface. Several configurations of anodes are described for obtaining the constant current density.

FIELD OF THE INVENTION

[0001] This invention relates to the field of electroplating metals ontoa semiconductor wafer.

PRIOR ART

[0002] In the fabrication of integrated circuits, metal layers are oftenformed on semiconductor wafers as part of a process for formingconductive lines. More recently, the electroplating of copper is used ina damascene process because of the high conductivity of copper whencompared, for instance, to aluminum.

[0003] As shown in FIG. 1, in the process for electroplating copper, acopper electrode (an anode) 10 is disposed above a surface 14 of asemiconductor wafer 13. A second electrode 12 is clamped about the outeredge of the wafer 13 and provides a conductive path to the firstelectrode through a barrier layer or seed layer formed on surface 14.When a potential 16 is applied between the electrodes 10 and 12, currentflows between the electrodes. When this occurs, copper moves from theplating solution to the surface 14, thereby plating the surface 14 withcopper.

[0004] The deposition rate of copper in a given area on the surface 14is directly related to the current density, that is, the more currentthe more copper is deposited. The seed layer or barrier layer on thesurface 14 has a relatively high resistivity. As a result, the currentpath between electrode 10 to electrode 12 follows paths of differentresistance depending upon where on the wafer surface the current entersthe barrier or seed layer as it moves toward the electrode 12. The path,for example, that includes the center of the wafer has more resistancebecause the current must travel the full radius of the wafer. Incontrast, the path nearer the electrode 12 and electrode 10 has arelatively short path, and consequently, encounters a lower resistance.For this reason, the current flow between the anode and the wafersurface 14 is not uniform across the surface of the wafer. Less currentflows in the center of the wafer and more current flows toward theperiphery of the wafer per unit area. This causes the thickness of thecopper layer to be thicker near electrode 12 and thinner in the centerof the wafer.

[0005] The generally concave shaped copper layer formed on the wafer 13is made uniform by polishing the layer using, for instance,chemical-mechanical polishing (CMP). There are numerous disadvantageswith depositing a non-uniform layer and polishing it, some of thesedisadvantages are discussed later.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a schematic of a prior art electroplating apparatus.

[0007]FIG. 2 is a schematic of an electroplating apparatus in accordancewith one embodiment of the present invention.

[0008]FIG. 2A is a plan view of a segment of the anode of FIG. 2.

[0009]FIG. 2B is a plan view of a segment of an alternate embodiment ofan anode.

[0010]FIG. 3 is a schematic of another embodiment of an electroplatingapparatus.

[0011]FIG. 4 is a schematic of yet another embodiment of anelectroplating apparatus in accordance with the present invention.

DETAILED DESCRIPTION

[0012] Electroplating methods, apparatuses, and anodes, particularly foruse in forming a metal layer on a semiconductor wafer are described. Themethods, apparatuses, and anodes are described for the formation of acopper layer in a damascene process. It will be apparent that thepresent invention may be used for forming other layers. Moreover, in thefollowing description, numerous specific details known in the art, suchas for the formation of barrier and seed layers in a damascene process,and electroplating chemistry are not set forth in detail in order not toobscure the present invention.

[0013] With some of the methods, apparatuses, and anodes describedbelow, a relatively uniform current density is achieved between theanode of the electroplating apparatus and the surface of thesemiconductor wafer upon which the layer is plated. Several anodeconfigurations are discussed below for providing this uniform currentdensity. This uniform current provides a layer of relatively uniformthickness. Thus, for instance, in the formation of a copper layer in adamascene process, less polishing is required to provide a flat surfaceand to define the inlaid conductors. This more readily allows the use ofmechanically weaker, lower k dielectrics. The need for a hard mask maybe eliminated in some processes because of the uniform layer. Lesspolishing is needed which shortens the processing time. Other advantageswill be apparent to one skilled in the art.

[0014] Referring first to FIG. 2, a wafer 20 clamped within a firstelectrode 22 is illustrated. The clamp 22 of FIG. 2 may be similar tothe clamp 12, shown in the prior art apparatus of FIG. 1. A layer isformed on the surface 21 of the wafer, such as a copper layer.

[0015] While not specifically shown in FIG. 2, prior to placing thewafer 20 in the electrode 22, a barrier layer or seed layer is formed onthe surface 21, particularly where a damascene interconnect structurefor an integrated circuit is formed. The barrier layer is often atantalum or TaN layer used to prevent the subsequently formed copperfrom diffusing into the interlayer dielectric (ILD) and into otherregions of the integrated circuit. In some instances, a seed layer isformed on the barrier layer, such as by sputtering copper, to increasethe conductivity of the barrier layer.

[0016] The barrier layer, even with the seed layer, are not particularlyconductive. Consequently, as described above, there is a significantvoltage drop associated with the current flow between the firstelectrode 22 and the anode 26 as a function of the distance the currentmust travel.

[0017] The anode 26 of FIG. 2 comprises a generally disc-like memberhaving a pair of parallel surfaces, one surface of which faces thesurface 21 of wafer 20, as illustrated in FIG. 2. A plurality of rods,such as rod 27, are disposed between the faces of the anode 26 and aregenerally perpendicular to the surface 21 of the wafer 20. The rods areall of the same diameter for the illustrated embodiment. For copperplating, these rods are fabricated from copper. The body of the anode 26may be formed from an insulative member such as Teflon, with the rods 27inserted into the Teflon member.

[0018] In one embodiment, the density of the rods is greater at thecenter of the anode 26 than it is at a distance away from the center.For instance, as shown in FIGS. 2 and 2A, the rods indicated by thebracket 28 are more densely disposed than those indicated by the bracket29. Any number of different densities may be used even though in FIG. 2,only three different densities are shown.

[0019] The rods are coupled to a source of potential with respect to theclamp/electrode 22. In FIG. 2, the rods of each density are coupled to adifferent potential. The different potentials provide an additionalparameter that can be varied to achieve a uniform current density. Forinstance, the rods 28 are coupled to a potential V₃ and the rods 29 arecoupled to a potential V₄. To provide a more uniform current flow, V₃may be a higher potential than V₄. In FIG. 2, five different potentialsare illustrated. Again, any number of different potentials may be used.Moreover, the potentials V₁ and V₅ may be equal to one another, andsimilarly, the potentials V₂ and V₄ may be equal to one another. Thesevoltages such as V₁ and V₅ are illustrated as being different from oneanother. Different potentials may be used to balance the currents atdifferent regions of the wafer surface equidistant from the center.

[0020] The anode 26 of FIG. 2 may also be used where all the rods arecoupled to the same potential since the different densities of theconducting rods will tend to cause a uniform current density across thesurface 21 of wafer 20.

[0021] In another embodiment, the rods may be of uniform density withinthe anode 26 and the different voltages shown in FIG. 2 may be used toachieve the uniform current density between the anode and the wafersurface 21. The same result can be achieved by varying the diameter ofthe rods. For instance, a higher “density” of rods can be obtained byincreasing the diameter of the rods in one region when compared to otherregions of the anode.

[0022] In yet another embodiment shown in FIG. 2B, rather than rods,annular, ring-like conductive elements 33 fabricated from, for instancecopper, may be used for the anode 30. The elements are separated fromone another by annular shaped Teflon members 32, as an example. Theannular conductive elements may have a uniform density or may have agreater density towards the center of the electrode similar to thedispersal shown for the rods of FIG. 2. For the embodiment of FIG. 2B,the conductive rings 33 are uniformly spaced. A uniform current densityacross the surface of the wafer is obtained by using different voltages.The rings closer to the center of the anode 30 receive a higherpotential than the potential applied to the rings with a largerdiameter.

[0023] For all the embodiments, including the prior art, a reductionreaction occurs at the wafer and an oxidation reaction occurs at theanode. The wafer is negative relative to the anode and the Cu⁺² ions inthe plating solution, in which the electrode and wafer are submerged,are attracted to the wafer surface.

[0024] In the embodiment of FIG. 3, the anode 36 has a lenticular shape,more specifically, it has a convex surface facing the surface 35 of thewafer 34. When a potential is applied between the clamping electrode 38and the copper electrode/anode 36, the voltage gradient between thewafer 34 and the anode is greater at the center of the wafer than atdistances on the surface 35 spaced apart from the center. Thiscompensates for the fact that there is more resistance between theelectrodes at the wafer center. The shape of the anode 35 is selected toprovide a uniform current density between the surface 35 and theelectrode 36. Again as mentioned, this provides a uniform thickness of alayer plated onto surface 35.

[0025] In the embodiment of FIG. 4, a wafer 41 is again held by anelectrode/clamp 52. This time, however, the surface 41 that is to beplated is facing downward for the illustrated embodiment. A volume 50 isdefined by, for instance, a cylindrical cell 42 having an inlet 52 andan outlet 53. An anode, such as a copper electrode 51, is disposedwithin the volume 50. A voltage is applied between the anode 51 and theelectrode 52. The cylindrical cell 42 is moved relative to the surface41 such that the outlet 53 can be, in effect, swept over the entiresurface 41 of the wafer 40.

[0026] The cell 42 is filled with a plating solution. In practice, whilenot illustrated, the outlet 53 is a relatively short distance from thesurface 41. Enough space is provided to allow the plating fluid toescape from a gap between the surface 41 and the outlet 53, as shown bythe arrows. The plating solution first flows upward and then escapesthrough the gap between the cell and the wafer surface 41. Once theliquid has escaped the cell, it drains downward and away from thesurface of the wafer. Consequently, only a fraction of the wafer surfaceis exposed to the electroplating solution at any given time.

[0027] The entire surface 41 can be electroplated by moving the cellrelative to the surface 41. The anode voltage 60 is varied as theelectroplating cell is moved. More voltage is applied near the centerregion than at the region near the electrode 52. This voltage variationprovides a constant current density, and consequently, a constantplating rate. This results in a layer of uniform thickness.

[0028] Alternatively, the voltage 60 may remain constant and the rate atwhich the cell 40 is moved can be varied. For instance, the cell can bemoved at a slower rate near the center of the wafer than at theperiphery of the wafer. This again allows for a layer of uniformthickness since the plating rate at the center will be slower than atthe periphery because of the added resistance at the center from thebarrier or seed layer. A combination of the varied voltages and variedrate of movement can be used.

[0029] Thus, the method of the present invention is to provide an anodethat results in a uniform layer being formed on a wafer surface. In somecases, as shown above, this is achieved by having a greater voltage dropbetween the anode and wafer surface at the peripheral regions of thesurface. For instance, this occurs with the anode of FIG. 3. In otherinstances, more sources of current are provided near the center of thewafer than at the periphery, such as shown in FIG. 2. A uniform currentdensity is also achieved in some embodiments by having differentvoltages applied to different conductive elements in the anode, again toprovide a uniform current density between the wafer surface and anode.In the embodiment of FIG. 4, the uniform layer thickness is obtained bymoving an anode and either changing the rate of movement and/or thevoltage applied to the anode as it is moved.

What is claimed is:
 1. A method for electroplating a wafer comprising: connecting a first electrode on the periphery of a wafer to provide an electrical path to a surface of the wafer; and applying a potential with respect to the first electrode to a second electrode disposed adjacent to the surface of the wafer, such that the potential between the second electrode and the surface of the wafer varies across the surface of the wafer.
 2. The method of claim 1, wherein the variation results in a substantially uniform current density between the second electrode and the surface of the wafer.
 3. The method of claim 2, wherein the second electrode comprises copper.
 4. The method of claim 3, including forming a barrier layer or seed layer on the surface of the wafer before connecting the first electrode to the wafer.
 5. A method for electroplating a semiconductor wafer comprising: applying at least one potential to an anode disposed adjacent to a surface of the wafer such that the current flow between the surface of the wafer and the anode is substantially uniform over the surface of the wafer; and forming a layer of relatively uniform thickness on the surface of the wafer as a result of the uniform current flow.
 6. The method of claim 5, including forming a barrier layer on the surface of the wafer prior to the applying of at least one potential to the anode.
 7. The method of claim 5, including forming a seed layer on the surface of the wafer prior to the applying of at least one potential to the anode.
 8. The method of claim 5, wherein the layer includes copper.
 9. The method of claim 5, wherein during the forming of the layer, the voltage drop between the anode and the surface of the wafer is greater at the center of the wafer than it is at distances away from the center.
 10. An apparatus for electroplating a wafer comprising: a conductive clamp for engaging the periphery of a wafer; and an anode disposed above the wafer, comprising a plurality of conductive elements disposed in an insulative member, the conductive elements being coupled to at least one source of potential.
 11. The apparatus of claim 10, wherein the conductive elements comprise a plurality of rods having a uniform density in the insulative member.
 12. The apparatus of claim 10, wherein the conductive elements comprise a plurality of rods having a higher density in a center of the anode when compared to a distance away from the center of the anode.
 13. The apparatus of claim 10, wherein the conductive elements include a plurality of concentric, annular members.
 14. The apparatus of claim 10, wherein a plurality of voltages are applied to the conductive elements with a higher voltage being applied to conductive elements disposed at a center of the anode, when compared to conductive elements disposed at a distance away from the center of the anode.
 15. The apparatus of claim 10, wherein the conductive elements comprise copper.
 16. The apparatus of claim 11, wherein the conductive elements comprise copper.
 17. The apparatus of claim 12, wherein the conductive elements comprise copper.
 18. The apparatus of claim 13, wherein the conductive elements comprise copper.
 19. The apparatus of claim 14, wherein the conductive elements comprise copper.
 20. An apparatus for electroplating a wafer comprising: a conductive clamp disposed about the periphery of the electrode, a lenticular shaped anode disposed above a surface of the wafer such that the anode is closer to the wafer at a center of the wafer when compared to the periphery of the wafer.
 21. The apparatus of claim 20,wherein the anode is copper.
 22. An apparatus for electroplating a wafer comprising: an electrode disposed about the periphery of the wafer; a cell having an inlet and an outlet facing a surface of the wafer, the cell carrying an electroplating fluid and being movable such that the outlet of the cell, sweeps over substantially the entire surface of the wafer; an anode disposed within the cell; and a source of potential applied between the electrode and the anode.
 23. The apparatus of claim 22, wherein the source of potential is varied as the cell moves.
 24. The apparatus of claim 23, wherein the rate at which the cell moves is varied. 